How do I gain weight fast and healthy?

I am 18 years old, 6'3" and used to weigh a lean 165, but in the past 2 months or so i lost 25 pounds from mono. During my illness, I had virtually no appetite every day, so now I believe my stomach has shrunk to almost nothing because it is almost impossible to eat more than a slice of bread without feeling like I need to throw up. How do I put that weight back on quickly if I still don't have much of an appetite?